Yes, but then you have to add the memory used for the data held within each process. In the case of the app in that book, it would be the data for each Islands game.
But then let's say you implement a Blog server where every user, post, comment, is held into memory at once - how much data is this going to need? Can we afford it or should we implement a way to unload selectively processes from memory?
I've expressed this in the context of my own app in this other thread, but it would apply to any real-world example. For example, this forum, can we hold all of its topics, comments, users, etc., entirely in RAM?